Variability of the Canary Current Diagnosed by Inverse Box Models

Abstract

Four hydrographic cruises carried out between ~26.5 and 31°N in the eastern North Atlantic Subtropical Gyre in fall (2016 and 2017) and spring (2017 and 2018) are used to identify water masses and infer oceanic circulation. Geostrophic velocities are initially adjusted by referencing them to data from a Lower Acoustic Doppler Current Profiler (LADCP) and later to velocities estimated with an inverse box model. The distribution of the intermediate water masses (700 to 1,400 m depth) varies seasonally. Antarctic Intermediate Water (AAIW) comprises the largest contributor to the seasonal cycle in the intermediate water masses. Circulation of the Canary Current (CC) differs in fall and spring. In fall, the CC flows southward through the western islands and recirculates south of the archipelago, subsequently flowing northward through the passage between the eastern islands and Africa. North of Lanzarote, the recirculated CC intensified as it is joined by a southeasterly branch of the CC north of Lanzarote. In spring, the net transport of the CC is southward. High interannual variability in mass transport is evident in both spring and fall as a result of the position of the current, with its easternmost (westernmost) position found in spring (fall) 2018 (2016). At intermediate levels, highly variable northward/southward transport is apparent in fall over the African slope, with the Intermediate Poleward Under Current (IPUC) only present in 2017.

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

“…Western boundary density anomalies were thus chosen as the independent variables representing each layer, with the exception of AAIW. The seasonal cycle of the AMOC is driven largely by seasonality at the eastern boundary (Chidichimo et al, 2010;Pérez-Hernández et al, 2015). The annual maximum northward transport at the eastern boundary and the AMOC occur around October (Vélez-Belchí et al, 2017) and is driven by changes in the circulation of the Canary Current (Casanova-Masjoan et al, 2020;Hernández-Guerra et al, 2017) and at intermediate depths (700-1400 dbar) by seasonal changes in the Intermediate Poleward Undercurrent (Hernández-Guerra et al, 2017;Vélez-Belchí et al, 2017).…”

“…The annual maximum northward transport at the eastern boundary and the AMOC occur around October (Vélez-Belchí et al, 2017) and is driven by changes in the circulation of the Canary Current (Casanova-Masjoan et al, 2020;Hernández-Guerra et al, 2017) and at intermediate depths (700-1400 dbar) by seasonal changes in the Intermediate Poleward Undercurrent (Hernández-Guerra et al, 2017;Vélez-Belchí et al, 2017). Eastern boundary density anomalies have maximum subsurface variability around 1000 dbar (Chidichimo et al, 2010), so the AAIW layer was represented by an eastern boundary density anomaly between 800 and 1100 dbar. The multiple linear regression Eq.…”

“…As Longworth et al (2011Longworth et al ( ) documented, between 1980Longworth et al ( and 2017 there are many more hydrographic profiles close to the western boundary than the eastern. Since the eastern boundary density anomaly shows strong seasonal variability (Pérez-Hernández et al, 2015;Chidichimo et al, 2010), replacing it in the model with a monthly climatology from the RAPID eastern boundary data, as described in the previous Sect. 2.3, allows us to use all available western boundary profiles, although at the cost of losing a little of the explained UMO transport variance.…”

Response to comments by anonymous reviewer #1

“…The seasonal cycle of the AMOC is driven largely by seasonality at the eastern boundary (Chidichimo et al, 2010;Pérez-Hernández et al, 2015). The annual maximum northwards transport at the eastern boundary and the AMOC occur around October , and is driven by changes in the circulation of the Canary Current (Casanova-Masjoan et al, 2020;, and at intermediate depths ( 700-1400 dbar) by seasonal changes in the Intermediate Poleward Undercurrent . Eastern boundary density anomalies have maximum sub-surface variability around 1000 dbar (Chidichimo et al, 2010), so the AAIW layer was represented by an eastern boundary density anomaly between 800 and 1100 dbar.…”

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline by Emma L. Worthington et al.

“…, several papers have appeared dealing with the seasonal cycle of the AMOC and the eastern boundary of the NASG. Pérez-Hernández et al(2015), Vélez-Belchí et al(2017), Hernández-Guerra et al(2017)andCasanova-Masjoan et al (2020), among others, have found a seasonal behaviour of the Canary Current in the Lanzarote Passage that explains the seasonal cycle of the AMOC. I think these papers deserve at least a brief comment in the manuscript.…”